1. Field of the Invention
The present invention generally relates to an organic EL device, and more particularly to a water-capturing agent for inhibiting the generation and growth of a darkspot of a non-light emitting portion.
2. Description of the Prior Art
Generally, an organic EL device (organic electroluminescent device) has a light-emitting structure of laminate comprising an organic EL device sandwiched between an anode and a cathode which is a thin film containing a luminescent organic compound. A hole and an electron are injected into the thin film containing a luminescent organic compound and are re-combined to generate an exciton. When the exciton is deactivated, it emits light (fluorescence, phosphorescence). An organic EL device is a natural light device taking advantage of such fluorescence or phosphorescence.
One of the most significant problems of the organic EL device is an improvement in the lifetime of a light-emitting portion. The cause of a short lifetime is the generation of a non-light emitting dot known as a “darkspot”. The non-light emitting dot grows with the elapsed time of lighting and a non-light emitting area enlarges gradually and luminance decreases gradually. When the diameter of the non-light emitting portion grows up to tens of μm, it is confirmed visually. That is to say, when the diameter of the non-light emitting portion grows up to tens of μm, the lifetime of EL device nearly runs out. The main cause of the generation of the darkspot is that an organic EL layer constituting the organic EL device reacts with moisture or oxygen in a sealed container and the darkspot generates and grows partially.
It is, therefore, necessary to reduce to the utmost the moisture in a sealed container in which is placed an organic EL device. It is particularly important to refine an organic material used for a light emitting portion in a state that no moisture exists in the organic material. Further, several methods have been proposed in order to reduce to the utmost the moisture in a sealed container. For example, when the inside of a vacuum chamber used for forming an electroluminescent layer on a substrate is manufactured, or when a sealing cap is put over a light emitting portion formed on a substrate, an effort has been made to reduce to the utmost the moisture in the sealed container. That is to say, the manufacture of an EL organic device has been carried out in a dry process. However, the moisture can not be completely removed from such conventional manufacturing processes and hence the generation and growth of darkspots can not be completely inhibited.
As described above, the most important problem to be solved for an organic EL device is to eradicate the darkspot by removing the moisture completely in a sealed container or to attempt to prolong the lifetime of the device by stopping the growth of small darkspots. In a commercially available organic EL device, the afore-mentioned problem has been solved by placing an inorganic drying agent as a drying means in a container and sealing it. Such a technique is relevant to the present invention and is publicly known as described in Unexamined Patent Publications No. 2002-267138.
Further, a conventional organic drying means involves a powder which is a disadvantage. In order to solve such a disadvantage, a technique has been proposed for forming effectively a water-capturing means capable of inhibiting the generation and growth of the darkspot in the peripheral portion of an organic EL device. Such a water-capturing means has a water-capturing layer of film made of an organometallic compound having high reactivity with moisture, effectively captures the moisture, and does not contain oxygen. Such a technique is described in Unexamined Patent Publications No. 2002-33187.
FIG. 2 is a partly diagrammatic sectional view showing a sealing structure of an organic EL device. As shown in FIG. 2, an organic EL device has a structure in which the anode electrode 55 and cathode electrode 56 are placed opposite each other on the glass substrate 52 made of glass etc., and the organic light-emitting layer 54 of the organic EL device 51 is sandwiched between the anode electrode 55 and cathode electrode 56. A recess 59 formed in the sealing cap 53 is filled with powder of barium oxide (BaO) as the water-capturing agent 57. The cap is fixed with the water-permeable tape 60, such that the powder does not scatter into a tube. The sealed tube is filled with dried nitrogen air, and is fixed to the substrate 52 with the adhesive 58 of epoxy resin in order to prevent the moisture from invading from the outside.
As shown in FIG. 3, the organic EL device 71 has a structure in which the anode electrode 75 and cathode electrode 76 are placed opposite each other on the glass substrate 52, and the organic light-emitting layer 74 of the organic EL device 71 is sandwiched between the anode electrode 75 and cathode electrode 76. The organic light-emitting layer 74 has a three-layered structure comprising the hole-injecting layer 74a, the hole-transporting layer 74b and the light-emitting layer-and- electron-transporting layer 74c. The organic light-emitting layer 74 is placed in a sealed container comprising the glass substrate 72, the sealing cap 73 and the sealing portion 78. The water-capturing agent layer 77 is place in the sealed container in order to prevent the organic EL light-emitting layer 74 of the organic EL material from being contaminated with the moisture. The water-capturing agent layer 77 is composed of a thin film of an organometallic complex compound having the chemical structural formula (2), (3) or (4) illustrated below:

The organometallic complex compound having the chemical structural formula (2), (3) or (4) is soluble in an organic solvent and liquid at the time of mounting. Such an organometallic complex compound is applied to the entire surface of the inside of the sealing cap 73 of a flat glass plate which has been subjected to a spot facing processing by means of a processing machine, such as a sandblaster etc., and the water-capturing agent layer 77 having a thickness of 10 μm is formed. The water-capturing agent layer 77 of this type requires neither tape 60 nor a recess formed in the sealing cap 73, which is different from the case where powder-type water-capturing agent is used. Accordingly, the entire thickness of the organic EL device can be made thinner.
However, a conventional water-capturing agent BaO (barium oxide) has several disadvantages as described below:
The handling of the conventional water-capturing agent BaO (barium oxide) involves problems in a work environment or market, because it is a deleterious substance. Since barium oxide is powder, it scatters while it is charged into a sealed container. That is to say, it is not easy to handle. Since the water-capturing agent is powder, it is necessary to place independently a space for sealing the water-capturing agent. Accordingly, there has been a problem making the entire thickness of the device thinner.
In the case of a conventional film-shaped water-capturing agent, it is necessary to remove solvents by heating in a process for forming the film. Accordingly, there has been a problem that a long time is required to form the film.
A drying system used for the film-shaped water-capturing agent requires an apparatus of large size. Hence, there has been a problem that the space for installing such an apparatus in a manufacturing place is large which decreases operation efficiency.